Summary: Peatlands store more carbon (C) than any other terrestrial ecosystem and as a C sink they are vital to mitigating climate change. The keystone of many peatland ecosystems is Sphagnum, a bryophyte genus of c. 350 species found on every continent except Antarctica. With climate change, many peatlands face increasing frequency and severity of drought. How Sphagnum responds to and recovers from drought will be key to sustaining peatlands over the coming decades. Here, we synthesise the latest evidence for how interactions of drought with Sphagnum affect peatland functioning. We discuss how Sphagnum traits, from the cellular to the community, control its ecohydrology and what changes occur during drought. We detail the effects of drought on Sphagnum C cycling and biochemistry, including photosynthesis, growth, respiration and methane (CH₄) fluxes. We also highlight drought resilience and tipping points for Sphagnum physiology and at the ecosystem level. The implications of Sphagnum drought responses for peatland hydrology, restoration and wildfires are also outlined. Finally, we identify knowledge gaps and propose some urgent questions which should be addressed in future research.

The rapid, effective restoration of degraded peatlands is urgently needed to reduce their current high levels of carbon loss. The re-introduction of Sphagnum moss, along with re-wetting, is key to returning carbon sequestration and retention capabilities to northern degraded bogs. Micropropagated Sphagnum has already been applied in large quantities, and more is planned, for restoration projects in Britain and parts of Europe. A comparison with wild-sourced Sphagnum material is therefore pertinent to demonstrate its safety and suitability for wide-scale application. Six Sphagnum species of both micropropagated and wild-sourced origin were assessed for photosynthetic capacity, nutrient content, form parity, chlorocyst size, and chloroplast numbers. Micropropagated Sphagnum had significantly higher light-saturated photosynthesis (P_max) rates, little color expression, an open growth habit, greater chloroplast numbers, and more numerous, smaller shoot apices than wild-sourced Sphagnum. Higher P_max rates were associated with a lower bulk density and higher tissue nutrient concentrations. Potentially, greater chloroplast numbers in micropropagated Sphagnum facilitate higher photosynthesis rates, driving rapid growth in early-stage plants, particularly in optimum moisture conditions. Micropropagated Sphagnum can be used confidently, propagated in large quantities, and will likely establish well on application to sites where re-wetting has already occurred, therefore making it highly beneficial for the restoration of degraded bogs.

Nitrogen (N) deposition significantly affects forest dynamics, carbon stocks and biodiversity, and numerous assessments of N fluxes and impacts exist in temperate latitudes. In tropical latitudes, however, there are few such assessments. In this study, we measured the inorganic N concentration (wet deposition) deposited in rainfall and rainfall pH throughout one year at the boundary of a forest reserve in Malaysian Borneo. We considered that the N deposition may be either from forest and agricultural fires or derived from agricultural fertiliser. Therefore, we determined the wind trajectories using the HYSPLIT model provided by NOAA, the location of fires throughout the landscape throughout one year using NASA’s FIRM system, and obtained the land use cover map of Malaysia and Indonesia. We then correlated our monthly cumulative wet N deposition with the cumulative number of fires and the cumulative area of oil palm plantation that wind trajectories arriving at our study site passed over before reaching the rainfall sampling site. At 7.45 kg N ha⁻¹ year⁻¹, our study site had the highest annual wet inorganic N deposition recorded for a Malaysian forest environment. The fire season and the cumulative agricultural area crossed by the winds had no significant effect on N deposition, rainfall N concentration, or rainfall pH. We suggest that future research should use ¹⁵N isotopes in rainfall to provide further information on the sources of N deposition in tropical forests such as this.

Much attention has been given to the carbon balance of peatland and heathland ecosystems and their role as global carbon stores. They are also important as buffers for atmospheric nitrogen (N) pollution, locking N into the soil and vegetation through tight nutrient cycling and preventing the leaching of soluble N into freshwater ecosystems. We compared mean annual soil exchangeable N, mineralisation and soil solution nitrogen at three contrasting ericaceous-dominated ecosystems: a lowland heath, an upland heath and an ombrotrophic raised bog at intermediate altitude, all of which were sites of long-term N-manipulation experiments. We expected that soil leachate N would be associated with soil C/N and total soil C, and that sites with higher C % and soil C/N would have greater ability to buffer N deposition before N saturation and leaching began. However, although soil solution N responded to N deposition at all the sites, we found that only the heathland sites were consistent with this expectation. The bog, with the highest C/N and largest C pool, was not the most strongly buffered. The upland heath was most effective at retaining N (extractable NH ⁴⁺ -N +3900 % from control) compared to the lowland heath (extractable NH ⁴⁺ -N +370 % from control) and the bog (extractable NH ⁴⁺ -N, +140–240 % from control). We concluded that the absence of a definable Calluna litter layer at the lowland heath and the bog, and the anoxic conditions at the bog, explained the earlier onset of leaching and that carbon and nitrogen cycles appeared more closely coupled in the heathlands but became decoupled at the bog due to the strong controlling effect of hydrology.